1. Field of the Invention
The invention relates to a method for fabricating semiconductor device, and more particularly, to a method of integrating thyristor and metal-oxide semiconductor (MOS) transistor on a semiconductor substrate.
2. Description of the Prior Art
Thyristors are switching applications having four layers, P1-N1-P2-N2 and three P-N junctions in series. An electrode defined as an anode is coupled to the external P1 layer. An electrode defined as a cathode is coupled to the external N2 layer. A gate electrode is coupled to the middle P2 layer. A thyristor having this structure commonly referred to as a silicon-controlled rectifier (SCR).
A characteristic of the thyristor is that the middle junction is reverse biased when positive voltage is applied to the anode and passive voltage is applied to the cathode, so there is no electrical current passing through the thyristor. However, when the positive voltage is applied to the gate, the thyristor enters a breakdown and the voltage of breakdown is referred to as the breakover voltage. When breakover voltage is reached, electrical current crosses the junction from the cathode to the anode and such electrical current is typically referred to as a holding current. When the thyristor is in a breakdown, the gate is not controlled by the thyristor and the electrical current is maintained until the circuit breaks off or the voltage becomes zero, and then electrical current stops. Therefore, the thyristor has a characteristic of holding voltage.
Thyristors are also bipolar devices having characteristics of bistable and negative differential resistance (NDR) and are widely applied in static random access memories (SRAMs). The memory having thyristors are often referred to as T-RAM.
Conventional T-RAM includes a vertical thyristor and a metal oxide semiconductor (MOS). The vertical thyristor is a thyristor having a structure of P1-N1-P2-N2 stacked from bottom to top. Even though current T-RAM has advantages of stable electrical current and higher thermal stability, it has been difficult to integrate T-RAM with MOS transistors as the formation of vertical thyristors typically require numerous polysilicon deposition processes. Consequently, more complicated processes are needed to complete the fabrication of these thyristors. Hence how to improve the current thyristor design while integrating these elements with device such as MOS transistors has become an important task in this field.